Using user defined function withing Update query in SQL Server - sql-server

Question

Good day!
I've used my user-defined function within an update statement in the following way:
UPDATE Users
SET
CurrentLevel = [dbo].[GetCurrentLevel](id),
Rate = [dbo].[GetRate](id),
LastUpdate = GETUTCDATE()
WHERE id IN (1,2,3)
And it gives me the error:
Subquery returned more than 1 value. This is not permitted when the
subquery follows =, !=, <, <= , >, >= or when the subquery is used as
an expression.
I always get only 1 value whenever I execute these functions outside this Update query.
The following code works as expected
DECLARE id_cursor CURSOR LOCAL FORWARD_ONLY FOR
SELECT id FROM Users
WHERE id IN (1,2,3)
OPEN id_cursor;
FETCH NEXT FROM id_cursor
INTO #id
WHILE ##FETCH_STATUS = 0
BEGIN
DECLARE #rate FLOAT = [dbo].[GetRate](#id);
UPDATE Users
SET
CurrentLevel = [dbo].[GetCurrentLevel](#id),
Rate = #rate,
LastUpdate = GETUTCDATE()
WHERE id = #id
FETCH NEXT FROM id_cursor
INTO #id
END
I do not understand why am I forced to use a cursor and update 1 row at time.
UPDATE.
This is not a trigger, but a stored procedure.
The user defined-functions both have return type FLOAT, which means that can not return multiple values by definition. Function code:
CREATE FUNCTION [dbo].[GetRate]
(
#userID int
)
RETURNS FLOAT
AS
BEGIN
DECLARE #discount INT = (SELECT TOP 1 [BonusSize] FROM Bonuses WHERE userID = #userID);
DECLARE #rate FLOAT = (SELECT SUM(s.Level * (i.Bonus + #discount))
FROM [Stats] s
JOIN [Orders] v
ON s.Level > 0 AND s.id = v.id AND v.userID = #userID
JOIN (SELECT id, SUM(ISNULL(Bonus,0)) as Bonus FROM Items GROUP BY id) i
ON i.id = v.id)
RETURN ISNULL(#rate/86400.0, 0);
END